Gate fringing effect based channel formation for semiconductor device

ABSTRACT

A memory device is described. Generally, the device includes a string of memory transistors, a source select transistor coupled to a first end of the string of memory transistor and a drain select transistor coupled to a second end of the string of memory transistor. Each memory transistor includes a gate electrode formed adjacent to a charge trapping layer and there is neither a source nor a drain junction between adjacent pairs of memory transistors or between the memory transistors and source select transistor or drain select transistor. In one embodiment, the memory transistors are spaced apart from adjacent memory transistors and the source select transistor and drain select transistor, such that channels are formed therebetween based on a gate fringing effect associated with the memory transistors. Other embodiments are also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/403,422, filed on Jan. 11, 2017, which is a continuation of U.S.patent application Ser. No. 14/179,316, filed on Feb. 12, 2014, now U.S.Pat. No. 9,570,458, issued on Feb. 14, 2017, which is a divisional ofU.S. patent application Ser. No. 12/368,023, filed on Feb. 9, 2009, nowU.S. Pat. No. 8,692,310, issued on Apr. 8, 2014, all of which areincorporated by reference herein in their entirety.

FIELD OF TECHNOLOGY

This disclosure relates generally to technical fields of semiconductormanufacturing.

BACKGROUND

A conventional NAND flash memory device 100 includes multiple NANDstrings of memory transistors. FIG. 1 illustrates two NAND strings ofthe conventional NAND flash memory device 100. To program a memorytransistor 102, a selected bitline 1 is grounded by passing a bitlineselect voltage 104 of 0 volt through the drain of a drain select (DS)transistor 1 to a node of the memory transistor 102, while a bitline 2and the rest of the bitlines are self boosted by coupling to aprogramming voltage 118 and/or a pass voltage 120 to inhibit theprogramming. A drain select (DS) line 108 and a source select (SS) line110 are coupled to a node of a DS voltage 112 and to a node of a SSvoltage 114, respectively. The supply voltage of the DS line 108 turnson the DS transistor 1 and maintains the connection of the bitline 1 tothe bitline select voltage 104, but it turns off the DS transistor 2,thus disconnecting the bitline 2 from the bitline unselect voltage 106.The SS voltage 114 coupled to the SS line 110 turns off both the SStransistor 1 and the SS transistor 2, thus isolating the two bitlinesfrom a node of a common source voltage 116 during the programming.

Furthermore, a wordline N coupled to the control gate of the memorytransistor 102 is applied by the programming voltage 118 of 18 voltswhile the unselected wordlines are applied by the pass voltage 120 of 11volts. Since the channel of the bitline 1 is coupled to the bitlineselect voltage 104 of 0 volt, it is maintained at the channel voltage of0 volt, whereas the channel potential of the bitline 2 is coupled up bythe programming voltage 118 and/or the pass voltage 120. For example,with the pass voltage 120 of 11 volts being supplied to the unselectedwordlines, the channel voltage of the bitline 2 may range between 6 to 9volts.

Before the programming takes place, the threshold voltage of the cell102 is about −2 volt. When the programming voltage 118 is applied, thehigh voltage of the programming voltage 118 causes the tunneling ofelectrons from the silicon substrate of the memory transistor 102 to thecharge trap layer of the memory transistor 102, thus increasing thethreshold voltage to a positive voltage of 1 volt, whereas the voltagedifferential between the silicon substrate and the floating gate of eachunselected cell is not large enough to cause the change in the thresholdvoltage of its respective transistor.

FIG. 2 illustrates an exploded view of a portion of the bitline 2 viewedacross Y-direction which includes the SS transistor 2, a memorytransistor 122 (e.g., coupled to an edge wordline, such as the wordlineN), and a memory transistor 124. The SS transistor 2 controls thebitline 2 for connecting to a common source 202. Furthermore, the SStransistor 2 is 200 nm in size, the transistors 90 nm in size, and thechannels 100 nm in size. The boosted junction potential between the SStransistor 2 and the memory transistor 122 may range between 6 and 9volts, where the gate of the SS transistor 2 is grounded. Provided thatthe channel voltage for unselected bitline 2 being 6 to 9 volts, thegate voltage of the memory transistor 122 being 18 volts, and the gatevoltage of the SS transistor 2 being 0 volt, there is a greatdisturbance with the band-to-band tunneling of the memory transistor122. That is, the memory transistor 122 is disturbed by an electron holepair (EHP) generation due to a gate induced drain junction leakage(GDIL) current.

The GIDL current arises in a high electric field under a gate-junctionoverlap region and a low gate to drain bias. The GIDL current occurswhen current flows from the junction 204 in direction to the substrate208 under the gate-junction overlap region, such as the overlap regionof the gate of the SS transistor 2 and the junction 204. The GIDL is dueto the formation of the depletion region and the region's high electricfield in presence of the low or negative bias in the gate of the SStransistor 2 (e.g., 0 volt), and the positive bias in the junction 204of the cell 122 (e.g., 6 to 9 volts). In the overlap gate-junctionregion, the high electric field creates electron-hole pairs (EHPs) whereelectrons through the barrier height are collected by the junction 204,and the holes (e.g., a hole 210) are collected by the substrate 208.When the electrons (e.g., an electron 212) generated due to the GIDLjump on a charge trapping layer 214 of the memory transistor 122, theelectrons may program the memory transistor 122, which is not selectedfor programming, thus resulting in a programming error.

As the chip size gets smaller, the smaller channel length may create ashort channel effect where the drain voltage of each transistor in thechip has more effect on the drain current than the gate to sourcevoltage has. Accordingly, the short channel effect may contribute to theoccurrence of the programming error due to the GIDL, which is anotherobstacle to the industry's effort for scaling down the chip size.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

An embodiment described in the detailed description is directed to aNAND flash memory device which comprises multiple NAND strings of memorytransistors, with each one of the memory transistors including a chargetrapping layer and a gate electrode formed on the charge trapping layer,where the memory transistors are formed close to each other to form achannel between an adjacent pair of the memory transistors based on agate fringing effect associated with the adjacent pair of the memorytransistors.

Another embodiment described in the detailed description is directed toa method for forming a NAND string of memory transistors which comprisesforming multiple charge trapping layers on a semiconductor substrate,and forming respective gate electrodes on the charge trapping layers,where the memory transistors are formed close to each other to form achannel between an adjacent pair of the memory transistors based on agate fringing effect associated with the adjacent pair of the memorytransistors.

As illustrated in the detailed description, other embodiments pertain todevices and methods that provide an improved fabrication process of aNAND flash memory device, and in particular, an omission of drain andsource region formation from the conventional methods of fabricating theNAND flash memory device. By forming memory transistors of the NANDflash memory device sufficiently close to each other, the channelsbetween adjacent pairs of the memory transistors can be formed based ongate fringing effects of their gate electrodes. As a result, thefabrication process of the NAND flash memory device can be simplifiedsignificantly. In addition, since there is neither source nor drainregion formed in the NAND flash memory device, the programming error dueto the GIDL current can be eliminated. Furthermore, since there is noneed to worry about the short channel effect with the elimination of thejunction region in the device, the NAND flash memory device can befurther scaled down.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments are illustrated by way of example and not limitationin the figures of the accompanying drawings, in which like referencesindicate similar elements and in which:

FIG. 1 illustrates two NAND strings of the conventional NAND flashmemory device.

FIG. 2 illustrates an exploded view of a NAND string of the conventionalNAND flash memory device of FIG. 1.

FIG. 3 illustrates an exploded view of a NAND string of an exemplaryNAND flash memory device, according to one embodiment.

FIGS. 4(A) and 4(B) illustrate process steps for fabricating the NANDstring of FIG. 3, according to one embodiment.

FIG. 5 is a process flow chart for forming a NAND string of an exemplaryNAND flash memory device, according to one embodiment.

Other features of the present embodiments will be apparent from theaccompanying drawings and from the detailed description that follows.

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the claims. Furthermore, in the detaileddescription of the present invention, numerous specific details are setforth in order to provide a thorough understanding of the presentinvention. However, it will be obvious to one of ordinary skill in theart that the present invention may be practiced without these specificdetails. In other instances, well known methods, procedures, components,and circuits have not been described in detail as not to unnecessarilyobscure aspects of the present invention.

Some portions of the detailed descriptions that follow are presented interms of procedures, logic blocks, processing, and other symbolicrepresentations for fabricating semiconductor devices. Thesedescriptions and representations are the means used by those skilled inthe art of semiconductor device fabrication to most effectively conveythe substance of their work to others skilled in the art. A procedure,logic block, process, etc., is herein, and generally, conceived to be aself-consistent sequence of steps or instructions leading to a desiredresult. The steps are those requiring physical manipulations of physicalquantities. Unless specifically stated otherwise as apparent from thefollowing discussions, is appreciated that throughout the presentapplication, discussions utilizing terms such as “forming,”“performing,” “producing,” “depositing,” or “etching,” or the like,refer to actions and processes of semiconductor device fabrication.

Briefly stated, other embodiments pertain to devices and methods thatprovide an improved fabrication process of a NAND flash memory device,and in particular, an omission of drain and source region formation fromthe conventional methods of fabricating the NAND flash memory device. Byforming memory transistors of the NAND flash memory device sufficientlyclose to each other, the channels between adjacent ones of the memorytransistors can be formed based on gate fringing effects of their gateelectrodes. As a result, the fabrication process of the NAND flashmemory device can be simplified significantly. In addition, since thereis neither a source nor a drain region formed in the NAND flash memorydevice, the programming error due to the GIDL current can be eliminated.Furthermore, since there is no need to worry about the short channeleffect with the elimination of the junction region in the device, theNAND flash memory device can be further scaled down.

FIG. 3 illustrates an exploded view of a NAND string of an exemplaryNAND flash memory device, according to one embodiment. The NAND flashmemory device includes multiple NAND strings (e.g., bitlines) of memorytransistors, and each memory transistor (e.g., a memory transistor 302,a memory transistor 308, etc.) includes a charge trapping layer (e.g., acharge trapping layer 304, a charge trapping layer 310, etc.) and a gateelectrode (e.g., a gate electrode 306, a gate electrode 312, etc.)formed on the charge trapping layer. In one embodiment, the memorytransistors (e.g., a memory transistor 302, a memory transistor 308,etc.) are formed close to each other such that a channel (e.g., achannel 314) is formed between an adjacent pair of the memorytransistors based on a gate fringing effect associated with the adjacentpair of the memory transistors. It is appreciated that the gate fringingeffect is an electric field leakage through a periphery of the gateelectrode (e.g., the gate electrode 306, the gate electrode 312, etc.)of each memory transistor. This effect becomes greater as the size ofthe NAND flash memory device becomes smaller.

Therefore, since the NAND flash memory device can form the channelbetween the adjacent memory transistors using the gate fringing effect,there is no need to form a source or a drain. In FIG. 3, the memorytransistors are separated from each other by approximately 40nanometers. In an alternative embodiment, the memory transistors can beseparated from each other by less than 40 nanometers. As a result, theNAND flash memory device can be scaled down significantly withoutaffecting its operation.

Similar to the NAND flash memory device 100 of FIG. 1, a source selectline is coupled to each one of the multiple NAND strings of memorytransistors, where the source select line comprises a source selecttransistor (e.g., a SS transistor 316) with a select gate at eachintersection of the multiple NAND strings and the source select line. Inaddition, the source select transistor 316 and an adjacent memorytransistor (e.g., the memory transistor 302) are formed close to eachother such that a first channel 318 is formed between the source selecttransistor 316 and the adjacent memory transistor based on gate fringingeffect associated with the source select transistor 316 and the adjacentmemory transistor. The source select transistor 316 and the adjacentmemory transistor are separated by approximately 40 nanometers or less.

It is appreciated that since there is neither source nor drain formed inthe semiconductor substrate of the NAND flash memory device asillustrated in FIG. 3, there is no overlap region of the gate of the SStransistor 316 and the junction. Thus, no GIDL current is generated inthe vicinity of the SS transistor 316. Therefore, there is nodisturbance due to an electron hole pair (EHP) generation since there isno GDIL current. Accordingly, the occurrence of a programming error tothe adjacent memory transistor (e.g., the memory transistor 302) can beeliminated since there is no EHP generation which causes the phenomenon.

FIGS. 4(A) and 4(B) illustrate process steps for fabricating the NANDstring of FIG. 3, according to one embodiment. In FIG. 4(A), a tunnelinglayer such as a tunnel oxide film 404 is formed on a semiconductorsubstrate 402. Next, a charge trap layer such as nitride film 406 isformed on the tunnel oxide film 404. Then, a top blocking layer such asa top oxide film 408 is formed on the nitride film 406. For example, thetunnel oxide film 404, the nitride film 406, and the top oxide film 408form a charge trapping layer or a floating gate of a NAND flash memorydevice. Furthermore, a polysilicon film 412 or metal film is formed onthe top oxide film 408 as a gate electrode.

FIG. 4(B) illustrates memory transistors (e.g., a memory transistor 414,a memory transistor 420, etc.) and a source select transistor 426 formedon the NAND string. It is appreciated that the memory transistors andthe source select transistors 426 may be formed by a variety of maskingand/or etching techniques. Each memory transistor includes a chargetrapping layer (e.g., a charge trapping layer 416, a charge trappinglayer 422, etc.) and a gate electrode (e.g., a gate electrode 418, agate electrode 424, etc.) formed on the charge trapping layer. In oneembodiment, the memory transistors comprise neither a source nor a drainsince a channel between the memory transistors can be formed based on agate fringing effect associated with the memory transistors. In order toform the channel based on the gate fringing effect, the memorytransistors need to be sufficiently close to each other. In oneexemplary implementation, the memory transistors are separated byapproximately 40 nanometers. In another exemplary implementation, theadjacent ones of the memory transistors are separated by less than 40nanometers. It is appreciated that the fabrication process illustratedin FIGS. 4(A) and 4(B) is significantly simpler than the conventionalfabrication techniques since steps for forming diffusion regions servingas a source region and/or a drain region, such as implanting impuritiesin the semiconductor substrate, can be eliminated.

In one embodiment, similar to FIG. 1, the source select transistor 426having a select gate is formed next to a memory transistor (e.g., thememory transistor 414). In addition, the source select transistor 426and the memory transistor are formed close to each other such that afirst channel is formed between the source select transistor 426 and theadjacent memory transistor based on a gate fringing effect associatedwith the source select transistor 426 and the adjacent memorytransistor. In one exemplary implementation, the source selecttransistor 426 and the adjacent memory transistor are separated byapproximately 40 nanometers. In another exemplary implementation, thesource select transistor 426 and the adjacent memory transistor areseparated by less than 40 nanometers. It is appreciated the spacebetween the source select transistor 426 and the adjacent memorytransistor may be same as the space between the adjacent pair of thememory transistors (e.g., the memory transistor 414, the memorytransistor 420, etc.) to simplify the fabrication process of the NANDflash memory device.

FIG. 5 is a process flow chart for forming a NAND string of an exemplaryNAND flash memory device, according to one embodiment. In operation 502,multiple charge trapping layers are formed on a semiconductor substrate.In operation 504, respective gate electrodes are formed on the chargetrapping layers, where the memory transistors are formed close to eachother such that a channel is formed between an adjacent pair of thememory transistors based on a gate fringing effect associated with theadjacent pair of the memory transistors. In addition, a select linecoupled to the NAND string of memory transistors is formed, where thesource select line comprises a source select transistor with a selectgate at an intersection of the NAND string of memory transistors and thesource select line.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A device comprising: a base layer; a first memorytransistor comprising: a first gate electrode; and a first chargetrapping layer disposed between the base layer and the first gateelectrode; and a second memory transistor comprising: a second gateelectrode; and a second charge trapping layer disposed between the baselayer and the second gate electrode; a source select transistorcomprising: a source select gate electrode; wherein the source selecttransistor, the first memory transistor and the second memory transistorare disposed in a NAND string; wherein the source select transistor isat one end of the NAND string, the first memory transistor is adjacentto the source select transistor and the second memory transistor isadjacent to the first memory transistor; wherein the first memorytransistor and second memory transistor are separated by a first width,the first width forming a first channel between the first memorytransistor and the second memory transistor, and the first channel isformed in a region of the base layer with no implanted impurities in thebase layer between the first memory transistor and the second memorytransistor; wherein the first memory transistor does not include adiffusion region serving as a source or a drain; wherein the secondmemory transistor does not include a diffusion region serving as asource or a drain; wherein the first channel does not include adiffusion region; wherein the first memory transistor and source selecttransistor are separated by a second width, the second width forming asecond channel between the first memory transistor and the source selecttransistor, and the second channel is formed in a region of the baselayer with no implanted impurities in the base layer between the firstmemory transistor and the source select transistor; wherein the sourceselect transistor does not include a diffusion region serving as asource or a drain; wherein the second channel does not include adiffusion region; wherein the first channel serves as a first sourceregion, a first drain region, or a combination thereof; and wherein thesecond channel serves as a second source region, a second drain region,or a combination thereof.
 2. The device of claim 1: wherein the baselayer comprises a semiconductor layer; and wherein the first channel isformed in a first region of the semiconductor layer between the firstmemory transistor and the second memory transistor.
 3. The device ofclaim 1, wherein the first width is less than 40 nanometers.
 4. Thedevice of claim 1, wherein the first width enables forming the firstchannel based on a gate fringing effect associated with the first memorytransistor and the second memory transistor.
 5. The device of claim 4:wherein the first charge trapping layer comprises: a first oxide film; afirst charge trap layer or a first charge trap layer stack; and a secondoxide film; wherein the second charge trapping layer comprises: a thirdoxide film; a second charge trap layer or a second charge trap layerstack; and a fourth oxide film; and wherein the first charge trap layeror the first charge trap layer stack is disposed between the first oxidefilm and the second oxide film and the second charge trap layer or thesecond charge trap layer stack is disposed between the third oxide filmand the fourth oxide film.
 6. The device of claim 5, wherein the firstcharge trap layer is a first nitride film or a nitride film stack andthe second charge trap layer is a second nitride film or a nitride filmstack.
 7. The device of claim 6, wherein the first gate electrodecomprises a first metal or a poly silicon film and the second gateelectrode comprises a second metal or a polysilicon film.
 8. The deviceof claim 1, wherein the first charge trapping layer is a first floatinggate of a NAND flash memory device and the second charge trapping layeris a second floating gate of the NAND flash memory device.
 9. The deviceof claim 1: wherein the base layer comprises a semiconductor layer;wherein the first channel is formed in a first region of thesemiconductor layer between the first memory transistor and the secondmemory transistor; and wherein the second channel is formed in a secondregion of the semiconductor layer between the first memory transistorand the source select transistor.
 10. The device of claim 1, wherein thefirst width is less than 40 nanometers and the second width is less than40 nanometers.
 11. The device of claim 1: wherein the first widthenables forming the first channel based on a gate fringing effectassociated with the first memory transistor and the second memorytransistor; and wherein the second width enables forming the secondchannel based on a gate fringing effect associated with the first memorytransistor and the source select transistor.
 12. The device of claim 1,wherein the first width and the second width are approximately equal.13. The device of claim 1, further comprising: a drain select transistorcomprising: a drain select gate electrode; and a drain select chargetrapping layer disposed between a semiconductor layer and the drainselect gate electrode; a third memory transistor comprising: a thirdgate electrode; and a third trapping layer disposed between thesemiconductor layer and the third gate electrode; wherein the thirdmemory transistor and the drain select transistor are disposed in theNAND string; wherein the drain select transistor is at the other end ofthe NAND string and the third memory transistor is adjacent to the drainselect transistor; wherein the third memory transistor and drain selecttransistor are separated by a third width, the third width beingsufficiently small to enable forming a third channel between the thirdmemory transistor and the drain select transistor; wherein the thirdchannel does not include a diffusion region; and wherein the thirdchannel serves as a third source region, a third drain region, or acombination thereof.
 14. The device of claim 13, wherein the firstwidth, the second width and the third width are approximately equal. 15.The device of claim 14, wherein the first width, the second width andthe third width are less than 40 nanometers.
 16. The device of claim 15,wherein the first channel is formed based on a gate fringing effectassociated with the first memory transistor and the second memorytransistor, the second channel is formed based on a gate fringing effectassociated with the first memory transistor and the source selecttransistor, and the third channel is formed based on a gate fringingeffect associated with the third memory transistor and the drain selecttransistor.
 17. A NAND flash memory device comprising: a plurality ofNAND strings of memory transistors, the plurality of NAND strings ofmemory transistors comprising a first NAND string of memory transistors;and a source select line; the first NAND string of memory transistorscomprising a base layer, a source select transistor, a first memorytransistor, a second memory transistor, and a third memory transistor;the first memory transistor comprising: a first gate electrode; and afirst charge trapping layer disposed between the base layer and thefirst gate electrode; and the second memory transistor comprising: asecond gate electrode; and a second charge trapping layer disposedbetween the base layer and the second gate electrode; the source selecttransistor electrically connected to the source select line comprising:a source select gate electrode; and a source select charge trappinglayer disposed between the base layer and the source select gateelectrode; wherein the source select transistor is at one end of thefirst NAND string of memory transistors, the first memory transistor isadjacent to the source select transistor and the second memorytransistor is adjacent to the first memory transistor; wherein the firstmemory transistor and second memory transistor are separated by a firstwidth, the first width forming a first channel between the first memorytransistor and the second memory transistor, and the first channel isformed in a region of the base layer with no implanted impurities in thebase layer between the first memory transistor and the second memorytransistor; wherein the first memory transistor does not include adiffusion region serving as a source or a drain; wherein the secondmemory transistor does not include a diffusion region serving as asource or a drain; wherein the first channel does not include adiffusion region; wherein the first memory transistor and source selecttransistor are separated by a second width, the second width forming asecond channel between the first memory transistor and the source selecttransistor, and the second channel is formed a region of the base layerwith no implanted impurities in the base layer between the first memorytransistor and the source select transistor; wherein the third memorytransistor does not include a diffusion region serving as a sourceand/or a drain; wherein the source select transistor does not include adiffusion region serving as a source or a drain; wherein the secondchannel does not include a diffusion region; wherein the first channelserves as a first source region, a first drain region, or a combinationthereof; and wherein the second channel serves as a second sourceregion, a second drain region, or a combination thereof.
 18. The deviceof claim 17: wherein the base layer comprises a semiconductor layer;wherein the first channel is formed in a first region of thesemiconductor layer between the first memory transistor and the secondmemory transistor; and wherein the second channel is formed in a secondregion of the semiconductor layer between the first memory transistorand the source select transistor.
 19. The device of claim 17, whereinthe first width is less than 40 nanometers and the second width is lessthan 40 nanometers.
 20. The device of claim 17, wherein the first widthenables forming the first channel based on a gate fringing effectassociated with the first memory transistor and the second memorytransistor; and the second width enables forming the second channelbased on a gate fringing effect associated with the first memorytransistor and the source select transistor.
 21. The device of claim 17,wherein the first charge trapping layer is a first floating gate of aNAND flash memory device, the second charge trapping layer is a secondfloating gate of the NAND flash memory device, and the source selectcharge trapping layer is a third floating gate of the NAND flash memorydevice.
 22. The device of claim 21: wherein the first charge trappinglayer comprises: a first oxide film; a first charge trap layer or afirst charge trap layer stack; and a second oxide film; wherein thesecond charge trapping layer comprises: a third oxide film; a secondcharge trap layer or a second charge trap layer stack; and a fourthoxide film; and wherein the first charge trap layer is disposed betweenthe first oxide film and the second oxide film, the second charge traplayer is disposed between the third oxide film and the fourth oxidefilm.
 23. The device of claim 22, wherein the first charge trap layer isa first nitride film, the second charge trap layer is a second nitridefilm, and the third charge trap layer is a third nitride film.
 24. Thedevice of claim 23, wherein the first gate electrode comprises a firstmetal or a poly silicon film, and the second gate electrode comprises asecond metal or a poly silicon film.